qemu: capabilities: Un-retire QEMU_CAPS_SDL

qemu: capabilities: Un-retire QEMU_CAPS_SDL

SDL graphics can be compiled out in qemu so we need to be able to know
whether the given qemu version support it.

Base the capability on the presence of the 'sdl' member in
'query-display-options' or imply it if 'query-display-options' is not
supported as we implied it before for all versions.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>